Astronomers had actually expected “a fine complexion” – but instead of dust-covered surfaces, the images of the asteroids Bennu and Ryugu showed very coarse-grained structures. The reason for this is a loss of the “powder” through an electrostatic effect, according to a study: The dust particles charge up, repel each other and thus begin to jump. Due to the low gravity of small asteroids, they can hop away, the simulations show. This form of material loss could even affect the movement of celestial bodies in the long term, the scientists say.
Neither water nor air gnaw at their structures, but the surface of asteroids is also affected by erosion processes. These are mainly due to their rotation. The surfaces are exposed to sunlight in relatively quick succession, then to shadow and finally to radiation again. The associated cycle of heating and cooling stresses the rock at the surface until it crumbles: the larger chunks break down into smaller and smaller chunks over time, eventually resulting in a dusty substance mostly made up of microscopic particles.
Inquiring look at rough asteroid surfaces
Therefore, it was to be expected that the surfaces of asteroids would be rather smooth and dusty. Studies have also confirmed this for the several-kilometre-wide asteroid Eros. But that was not the case for celestial bodies less than a kilometer in diameter: missions to the asteroids Ryugu and Bennu revealed surfaces that look like rough sandpaper and are strewn with large boulders. So far, it has remained unclear why these smaller asteroids are not covered by dust that should actually form as a result of erosion.
Scientists at the University of Colorado in Boulder have now devoted themselves to researching this mystery. In doing so, they pursued a concrete suspicion: from investigations into the characteristics of the dust that occurs on some celestial bodies, an effect is known that is referred to as “electrostatic turbulence”. The small particles absorb negative charges from the sun’s rays. They can add up, eventually creating an effect like two magnets repelling each other. The particles sometimes suddenly separate from each other – they jump. In some cases, the dust grains can be ejected at a speed of more than eight meters per second. “But no one had previously considered this process in the case of asteroid surfaces,” says co-author Xu Wang.
As part of their study, the scientists have now integrated the data from laboratory experiments into model simulations to investigate how electrostatic turbulence could affect asteroids. To do this, they created two hypothetical asteroids on the computer and investigated how dust grains would form there and then bounce around for hundreds of thousands of years. One of these fictional asteroids was similar in size to Ryugu, while the second was a rather large chunk several kilometers in diameter – similar to the asteroid Eros.
Loss due to electrostatic turbulence
The model simulations showed that on larger asteroids, the dust grains cannot gain enough speed to free themselves from gravity due to the electrostatic effect. On the smaller, Ryugu-like asteroid, however, things were different: “Gravity on smaller asteroids is so weak that it can’t stop drifting. As a result, the fine-grained regolith is lost,” says lead author Hsiang-Wen Hsu. In the model simulations, the smaller asteroid was almost completely freed of particulate matter within several million years. The Eros-like asteroid, on the other hand, remained dusty. According to the researchers, the loss of this protective layer could even lead to accelerated erosion. This may contribute to the particularly boulder-rich landscapes documented at Ryugu and Bennu.
The study thus now sheds light on what can be behind the characteristics of asteroids and how they change over time, the researchers say. The effect may also have another meaning: the loss of the dust could even affect the movements of the celestial bodies concerned. As the scientists explain, there is evidence that the dynamic effects caused by solar radiation differ between rocky and dusty structures. The features could thus have an impact on how asteroids travel through space.
As the scientists finally report, their study results will soon be put to the test. Because in less than three months, a NASA mission called the Double Asteroid Redirection Test (DART) will visit two smaller asteroids. “Then we will get new surface images to test our theory,” says Hsu.
Source: University of Colorado at Boulder, professional article: Nature Astronomy, doi: 10.1038/s41550-022-01717-9